A variety of techniques are used for measuring electrical current in a conductor. One technique is to allow some or all of the current to flow through a resistor and to measure the voltage-drop across the resistor. Ohm's law may then be used to calculate the current. If the resistor is in series with the conductor this will produce a voltage drop in the circuit, which is often undesirable. In addition, the resistance of the resistor is not easily altered, so dynamic change of the sensitivity of the sensor is difficult to achieve.
A second technique uses a current transformer, but this approach is expensive and can only be used with alternating currents. Another technique uses an open-loop or closed-loop Hall effect sensor. A current flowing through a conductor produces a magnetic field. The magnetic field may be concentrated by placing a magnetic core around the conductor. A Hall effect sensor placed in a gap in the magnetic core is used to measure the strength of the magnetic field. The magnetic core is required because of the low sensitivity of the Hall effect sensor and limits the miniaturization of the sensor.
Yet another technique uses a magneto-resistive sensor. A magneto-resistive sensor is a device that changes its resistance in the presence of a magnetic field. Like a Hall sensor, it can be used to measure the magnetic field produced by a current. However, it is more sensitive and may be used without a magnetic core to concentrate the magnetic field. A disadvantage of this approach is that the resistance change of a magneto-resistive material is not sensitive to polarity, so the direction or phase of the current flow cannot be determined. Further disadvantages include a limited linear range and poor temperature characteristics.